With a recent trend of electronic equipment becoming smaller in size and higher in density, double-sided circuit boards and multi-layer circuit boards are increasingly employed instead of conventional single-sided circuit boards on which electronic components are mounted, a development of a high-density circuit board capable of integrating as many circuits as possible on the circuit board is being made.
In a high-density circuit board, in place of a conventional method that has been widely employed for forming through-holes in a circuit board by drilling and making interstitial connection by a metal-plating, a method of manufacturing a circuit board having inner-via-structure capable of interstitial connection between wirings at the predetermined positions in higher density has been developed (for example, “Resin multi-layer circuit board ALIVH and its Application”, Masa Tachibana et al., May 1996, Surface Mounting Technology (published by Nikkan Kogyo Shinbun); and Japanese Patent Laid-open Publication 6-268345).
In such newly developed circuit board having inner-via-structure, a method of filling conductive paste into holes formed between the wirings is employed in order to make the interstitial connection.
Also, in a so-called build-up circuit board that has realized interstitial connection by a metal-plating of inner-via holes, to form a via-on-via structure in which inner-via-holes are stacked in the direction of thickness of the circuit board, it is necessary to smoothen the surface of the circuit board by filling a resin material paste or the like into the holes after the metal-plating.
As described above, in the production of high-density circuit boards of recent years, it is important to have a technology of filling conductive paste into the through-holes or a blind-holes.
However, in a high-density circuit board, with decrease in width of wiring and in size of electrode land, the size of the through-hole or the blind-hole is also reduced, and as a result, it becomes indispensable to optimize the filling conditions and also to control the viscosity of the paste for the purpose of precisely filling the paste into the holes.
Also, in order to mount a high-density circuit board in portable equipment for private use or the like, it is important to reduce the manufacturing cost, and there has been a strong demand for a method of manufacturing where circuit boards of as many as possible are produced with same quantity of paste.
An example of paste filling process in a conventional method of manufacturing a circuit board will be described with reference to FIG. 5A to FIG. 5F.
Board material 1 shown in FIG. 5A is a B staged prepreg sheet of after impregnating woven or non-woven fabric based on inorganic or organic fiber material such as glass fiber or aramid fiber with thermosetting resin material such as epoxy resin or the like, and release films 2 are bonded to both surfaces of the board material 1.
Further, through-holes 3 are formed in the board material 1 by a high-speed and small size drilling process using a laser beam or the like.
As shown in FIG. 5B, the board material 1 is placed on stage 10 by means of a board material transfer device (not shown).
On frame 9 is disposed paste 7 prepared by dispersing copper-based conductive particles into a binder component consisting of epoxy resin, hardener, solvent and the like.
Next, as shown in FIG. 5C, the board material 1 is slightly pressed by the frame 9, and then the paste 7 on the frame 9 at left in the figure is squeezed to the right in the figure by forward squeegee 4 installed in squeegee holder 6 and is filled into through-holes 3.
A squeegee elevation mechanism (not shown) and a squeegee pressing mechanism (not shown) are connected to the squeegee holder 6.
As shown in FIG. 5D, the mode is shifted to backward squeegee 5 on the frame 9 at right in the figure.
Subsequently, as shown in FIG. 5E, the paste 7 is squeezed to the left by the backward squeegee 5 and is filled into the through-holes again.
Next, as shown in FIG. 5F, the paste filling operation is completed when the paste 7 reaches on the frame 9 at left in the figure, and then the frame 9 moves up and the board material 1 is carried out by a board material transfer device (not shown), thereby obtaining the board material 1 with the paste 7 filled in the through-holes 3.
After that, the board material 1 filled with the paste 7 goes to a process where the release film 2 is removed therefrom. Also, the board material 1 is sandwiched between metal foils or the like and subjected to a hot press process to become a circuit board.
FIG. 6 shows a partially enlarged sectional view of the board material 1 filled with the paste 7 and carried out by the board material transfer device.
As shown in FIG. 6, the paste 7 is filled into the through-holes 3, and the surface of release film 2 is almost entirely coated with the binder component 17 and a very small amount of conductive particles 16 contained in the paste 7.
Such phenomenon takes place probably because the conductive particles 16 in the paste 7 are nearly adequately removed by the forward squeegee 4, while the binder component 17 in a state of being thinly coated on the release film 2 is hard to be removed and is not completely scraped off by the forward squeegee 5, thus remaining on the release film 2.
As a result, when comparing the component of paste 7 shown in FIG. 5B and the component of paste 7 shown in FIG. 5F after completion of the filling operation the latter is less in the quantity of binder component 17 in the ratio of conductive particles 16 and binder component 17.
The change in quantity is very slight, but it increases as the filling operation is continuously repeated on many board materials 1, causing the paste 7 to increase in viscosity due to reduction in quantity of the binder component 17, and finally there occurs a problem such that the paste filled into the through-hole 3 is insufficient or the paste 7 sticks to the release film 2 when the release film 2 is removed.
In order to avoid the occurrence of such problem, it is necessary to replace the paste with new one before the viscosity of paste 7 reaches the limit.
When the filling operation is performed by supplying new paste onto the manufacturing apparatus, hereinafter, a number of board materials treated between the supplying of the paste and a replacement of the paste is called a printable number.
The operation for replacement of the paste includes a considerable labor for supplying new paste and removing the old paste from the equipment, and a timing of executing the replacement depends on the number of through-holes 3 provided in the board materials 1 or variations in viscosity of the initial paste. Thus, the timing is not constant and it is very difficult to decide to execute the operation after how many board materials are printed.
From the viewpoint of manufacturing costs, it is desirable to repeat the filling operation until the paste viscosity reach to a usable limit before replacing the paste. However, as described above, since the timing of replacement is not constant, the replacement operation of the paste is actually performed a little earlier to the limit to allow some margin to avoid quality trouble.
Also, in the conventional circuit board manufacturing method described above, the paste is given conductivity for the purpose of interstitial connection of wiring, and therefore, it is difficult to make an amount of conductive particles in the paste lower than a specific value from the viewpoint of maintaining a conductivity. As a result, the paste has an increased viscosity, and it is also difficult to employ a method of increasing the printable number only by lowering the viscosity of the paste itself.
Further, the filling conditions such as a moving speed and an angle of forward squeegee 4 and backward squeegee 5 are designed in accordance with the physical properties or the like of paste 7, but it is difficult to control the filling conditions since the viscosity of paste 7 varies during the filling operation on many board materials 1.